One thing that hasn't been mentioned on this thread is the theory around viral load.
It seems like if you have a higher viral load (basically more virus in your system), then you're more likely to have a worse outcome (regardless of age). This is one of the reasons (possibly) that HCPs seem to have disproportionately more severe symptoms.
So if you are social distancing, you are more likely to only be infected by a small amount of the virus, and you are likely to have a milder disease, recover and develop immunity. If you are, say working in a school, and get exposed to several asymptomatic children over the course of the day, you'd potentially end up with a higher viral load, and be more likely to have a worse outcome.
If you have a low viral load, you may also be less likely to pass it on to others.
I'm not sure if there's enough evidence for this theory yet for it to be informing government policy, but it is worth bearing in mind.
Also, for those saying this is potentially harmless for the young, the figures from China show a deathrate of 0.2% for 11-18yos. That's about 2 kids per secondary school- that's an unacceptably high risk for a lot of people.
If we have a high number of deaths and lots of people off sick at once across all age groups (which would be the case with no social distancing/lockdown measures in place), then that would damage the economy too.
This is not just about the elderly.